We propose to examine the dynamics of cellular timekeeping using synchronous and non-synchronous animal cell cultures. The hypothesis that a short period, quantal clock is responsible for features of cell behavior such as temperature compensation and phase responsiveness will be tested using digital video microscopy, image processing and flow microfluorometry. The notion that the clock behaves as a limit cycle oscillator will also be explored. The role of cell size as a boundary condition in the expression of the oscillator and the possibility that this interaction leads to negative mother-daughter and positive sister-sister correlations in cell generation times will be tested. Entrainment of cell division by combinations of growth factors and by temperature shifts will be attempted using known cell cycle phase response behavior to these agents as a background. In vivo expression of entrainment of cell division by changing levels of growth factor will be explored using Mesocricetus auratus.